As semiconductor and integrated circuit (IC) technology has advanced, there has been a trend toward high-functionality IC components with numerous input and output (IO) pins, together with a demand for reduced chip size, weight, and power consumption. Consequently, as (IO) density increases, the pin size and spacing between pins must decrease.
To match the pin spacing of these ICs, there is a demand for printed circuit boards which will therefore have small, closely arranged solder pads. However, the reduction of the spacing between IC pins is currently happening at a greater rate than the corresponding solder pads on printed circuit boards. Consequently, there may be an interconnection technology gap for some modern devices.
To make such devices function, printed circuit boards may have extra routing layers to handle the pins of the ICs, or utilize fan-out packaging. This results in the package size of the IC being larger than the IC itself, which may limit system miniaturization. In addition to the desire for miniaturized devices, it is also desirable in some cases to construct these devices from a flexible, and not rigid, substrate.
One material now being used as a substrate from which to construct thin and flexible printed circuit boards is liquid crystal polymer (LCP). The molecules in LCPs have rigid, rod-like shapes, and maintain a crystalline order when in a liquid phase or when heated and melted. T. Zhang, W. Johnson, B. Farrell, and M. St. Lawrence, in their paper “The processing and assembly of liquid crystalline polymer printed circuits,” 2002 Int. Symposium on Microelectronics, 2002 discloses the construction of a printed circuit board using LCP as a substrate. A photoresist is first applied to a copper clad laminate, exposed, and developed to define a desired circuit pattern. The actual circuit is then defined by etching the exposed copper away. Holes or vias are created in the substrate via mechanical drilling. A desmearing step is then performed to remove debris from the vias or holes, thereby preparing the LCP material for metal deposition. A metallization step is next performed, and a typical solder mask is applied to the LCP substrate. Solder is then applied through the typical solder mask to complete the construction of the LCP printed circuit board.
While this design does allow for the creation of thin, flexible printed circuit boards, it still suffers from the same drawbacks as described above with respect to the attachment of ICs with closely spaced pins thereto. As such, additional methods of connecting ICs to printed circuit boards are needed.
The above described packaging issues may also be present when mounting a flip chip IC onto the printed circuit board. As with wire bond ICs, the board level interconnect for a flip chip device may comprise fan-out connectors due to limited feature resolution. Moreover, in these applications, under fill adhesive is formed under the flip chip IC to provide mechanical and environmental protection of the IC. Regardless of whether flip chip or wire bond ICs are used, as the packaging becomes more miniaturized, there is greater difficulty in controlling the placement and curing of the adhesive. For example, the adhesive may flow into sensitive areas, such as lithographically-defined solder mask openings and interconnect pads.
Moreover, with reduced pin sizes in state-of-the-art ICs comes a reduced standoff height between the flip chip ICs and the substrate. This may render capillary under fill techniques more difficult. One approach to manufacturing flip chip substrates is disclosed in U.S. Pat. No. 7,820,233 to Chen et al. The method comprises forming a patterned resist layer on a surface of a carrier, forming sequentially a first metal layer, an etching stop layer, and a second metal layer, removing the resist layer, and forming a patterned first solder mask, and then forming at least one first circuit build up structure thereon. The method also includes forming a patterned second solder mask on the circuit build up structure, removing the carrier, the first metal layer, and the etching-stop layer, and forming solder bumps on both sides of the circuit build up structure. A drawback to such an approach may be increased costs due to the high number of steps in the manufacturing process.